1. Field of the Invention
The present invention relates to a Ti-Mo alloy which exhibits outstanding resistance to pitting corrosion in an environment of high temperature and high pressure where there are bromide ions. The Ti-Mo alloy has good formability which is indispensable for materials constituting the chemical machines and equipment.
2. Description of the Prior Art
Titanium is superior in corrosion resistance, particularly in an environment where there are halogen ions. Because of this property, titanium has come into general use as a material for the process equipment which is exposed to such an environment. Nowadays titanium and titanium alloys are very important materials which support the entire industry. There are not any other materials than them that can be used in a severe environment where even stainless steel as the commonest anti-corrosion material is useless.
Nevertheless, the corrosion resistance of titanium is not always complete under any circumstances. It is often pointed out that titanium involves some problems in its corrosion resistance, due partly to the fact that titanium is used in environments under especially severe corrosive conditions.
What attracts more attention to the corrosion of titanium is localized corrosion that occurs and propagates locally rather than general corrosion that occurs all over the surface. What attracts special attention is crevice corrosion in an environment, particularly that of high temperature, where there are chloride ions. The next important problem is pitting corrosion in an environment where there are bromide ions. An example is the accident resulting from pitting corrosion in a high-temperature high-pressure reactor for the reaction catalyzed by a bromide.
Crevice corrosion occurs when a very narrow crevice is formed on the metal surface, whereas pitting corrosion does not necessarily require the presence of a crevice for its occurrence. Pitting corrosion occurs so locally that a penetrating hole may appear on the surface which is almost completely intact (say, more than 99%). Therefore, the occurrence of pitting corrosion is often overlooked, which leads to a sudden accident that takes place before an adequate measure is taken. It is fully recognized that it is very important to establish the means to prevent pitting corrosion. However, any means effective in preventing crevice corrosion cannot be used for the prevention of pitting corrosion because the two types of corrosion differ from each other in the mechanism of occurrence. Thus the development of a unique effective means is required.
The prevention of pitting corrosion may be achieved in two ways--the operation and control of the equipment and the improvement of the material itself. The first way is intended to make mild the operation conditions. There is naturally a limitation in doing so because it sacrifices the efficiency of the chemical process. The actual trend is rather contrary. The recent chemical process is performed under more severe conditions for corrosion than before. Such conditions often prevent the use of titanium. Under such conditions, an inhibitor may be added for the prevention of pitting corrosion. Anions such as sulfate, nitrate, and phosphate ions are effective as an inhibitor. The use of an inhibitor is not recommended freely because it contaminates the process and lowers the reaction yields.
The improvement of the material, mentioned above as the second way, is disclosed in Japanese Patent Laid-open No. 39785/1983 entitled "Method for treating the titanium surface with nitric acid", proposed by the present inventors. According to this method, the corrosion preventive treatment is carried out before the equipment is put to operation. The advantage of this method is that the process solution is not contaminated and the resistance to pitting corrosion is not affected by the kind of halogen ions. However, the use of a large amount of nitric acid (especially hot nitric acid) imposes some restrictions on this method in practical use. (The treatment is performed before or after the fabrication of the materials.)
It is thought that the pitting corrosion on titanium by halogen ions is initiated by the local anodic breakdown of the passive film formed on titanium, as will be described in detail later. Thus, the resistance of titanium to pitting corrosion should be evaluated by the breakdown voltage of the passive film. And it is considered that the higher the breakdown voltage, the greater the resistance to pitting corrosion. The breakdown voltage may be called the pitting potential (critical potential for occurrence of pitting corrosion).
It is known that the pitting potential can be increased when titanium is made into a nickel-containing titanium alloy. This holds true where the halogen ions are chloride ions. [See Desalination 3 269-279 (1967).] However, the present inventors found that the pitting potential of a nickel-containing titanium alloy is not so high as expected in an environment where there are bromide ions.
It was found that chloride ions and bromide ions behave entirely differently in pitting corrosion of a nickel-containing titanium alloy, although they are of the same category of halogen ions. In an attempt to develop a new alloy which resists pitting corrosion in an environment of bromide ions, the present inventors investigated how chloride ions and bromide ions differently affect the mechanism by which pitting corrosion occurs. They also investigated by using different alloys how the alloying element affects the prevention of pitting corrosion in an environment where there are chloride ions or bromide ions.
The present inventors investigated the formability of the alloy which is an important property to be considered when the alloy is used as the constituting material of the industrial chemical machines and equipment. They established the adequate quantities of Fe and O.sub.2 as impurities and the adequate conditions for annealing to render the alloy malleable.